1. Field of the Invention
The invention relates in general to a method for calibrating a spherical aberration (SA) compensation level of an optical drive, and more particularly to a method for calibrating the SA compensation level based on a peak-to-peak value of an S curve.
2. Description of the Related Art
Referring to FIG. 1, a schematic diagram of a conventional optical pickup head for reading a compact disk (CD) and digital video disk (DVD) is shown. From FIG. 1, it can be seen that the optical pickup head is composed of a laser diode 11, beam splitter 12, wavelength selector 14 and a focus object lens 15. When the optical pickup head is to perform a focusing operation, a laser beam is generated by the laser diode 11 to pass through the beam splitter 12 and the wavelength selector 14, and is then focused by the focus object lens 15 on the optical disk 16.
After focusing on the optical disk 16, the laser beam is reflected from the optical disk 16 to pass the beam splitter 12 and then arrives a photo detector (not shown) to generate optical signals such as focus error (FE) signals, track error (TE) signals and radio frequency (RF) signals. The optical detector is a charge-coupled device (CCD) or a complementary metal oxide semiconductor (CMOS).
As shown in FIG. 1, in order to precisely focus the laser beam on a data layer of the optical disk 16, the focus object lens 15 is designed to be movable in a moving range F. That is, the focus object lens can be controlled to get close to or apart from the optical disk. It can be determined according to the focus error signal whether the laser beam is focused on the data layer of the optical disk or not. Normally, the control chip in the optical drive can provide the focus actuator with different focus offsets in order to control the focus object lens 15 to reach an arbitrary position in the moving range F.
Therefore, with regard to the conventional CD and DVD optical storage technique, good focus quality can be achieved by just adjusting the position of the focus object lens 15 owing to that the spherical aberration (SA) of the focus object lens has not much influence on the focusing operation.
However, in terms of the advanced optical storage technique in a high definition-capable DVD (HD-DVD) or a blue-ray disk (BD), the storage density is increased by shortening the wavelength of the laser beam or enlarging the numeric aperture (NA) of the focus object lens, but at the same time the SA effect of the focus object lens becomes large and cannot be neglected, thereby enhancing the difficulty in precisely focusing. The focusing operation cannot be precisely performed by only adjusting the position of the focus object lens, but requires a SA compensation device as an aid. That is, to obtain an optimal focal point, it needs the SA compensation device to cooperate with the focus object lens.
Referring to FIG. 2, a schematic diagram of the optical pickup head used in the advanced optical storage technique is shown. The optical pickup head is composed of the laser diode 11, the beam splitter 12, a collimating mirror (CM) 13, the wavelength selector 14 and the focus object lens 15. The collimating mirror 13 is a SA compensation device, which can be replaced by a liquid crystal plate (LCP) or diffracted optical element (DOE). Therefore, when the optical pickup head is to perform the focusing operation, the laser diode 11 generates a laser beam to pass through the beam splitter 12 and become a parallel beam via the collimating mirror 13. Then, the parallel beam passes the wavelength selector 14 and is focused by the focus object lens on the optical disk 16.
After focusing on the optical disk 16, the laser beam reflected from the optical disk passes the beam splitter 12 to arrive a photo detector (not shown) to generate optical signals including the focus error signals, track error signals and RF signals.
As shown in FIG. 2, the collimating mirror 13 has a moving range S and can move in the moving range S to achieve an effect of dynamical SA compensation. That is, the collimating mirror 13 can be controlled to change its position to achieve the dynamical SA compensation. Normally speaking, the control chip of the optical drive can provide different SA compensation levels to control the collimating mirror to reach an arbitrary position in the moving range S. Of course, the dynamic SA compensation can also be achieved by providing different SA compensation levels on the LCP or DOE.
The focusing operation can be performed under every SA compensation level, but the good reading and writing quality cannot be achieved due to the SA influence. Therefore, only under the optimal SA compensation level, the SA has the minimum influence on the focal point, and thus the optimal reading and writing quality can be provided.
For this reason, in order to obtain the optimal SA compensation level, the conventional method selects a number of SA compensation levels for the focusing and tracking operations, and monitors performance of the HF jitters or push-pull amplitudes to determine the optimal SA compensation level. However, in addition to the SA factor, the focus offset also influences the HF jitters and push-pull amplitudes. Therefore, the optimal SA compensation level determined by using only one control variable would not be precise.
Referring to FIG. 3, a curve diagram of functional relation between the SA compensation levels and HF jitters and functional relation between the SA compensation levels and push-pull amplitudes is shown. From the two curves, it can be seen that under the same system, when the SA compensation level is 800, although the HF jitter has a minimum value, the push-pull amplitude does not reach the maximum. That is, when the SA compensation level is 800, the data reading and writing quality is the best due to minimizing of the HF jitter, but the push-pull amplitude, which is not the maximum, will cause that the servo control system in the optical drive cannot be controlled easily. Comparatively, when the SA compensation level is 1500, although the maximum push-pull amplitude can be obtained, the HF jitter does not reach the minimum. That is, when the SA compensation level is 1500, the servo control system in the optical drive can be better controlled due to maximizing of the push-pull amplitude, but the HF jitter, which is not the minimum, will cause the reduction of data reading and writing quality. Both situations result from that the optimal SA compensation level cannot be obtained since the SA compensation level is set as the only control variable in the above two curves without considering the factor of focus offset. Furthermore, the monitoring and calculation of the HF jitters or push-pull amplitudes can be performed only after the optical drive completes the tracking operation, which wastes much more time in the calibration of SA compensation level.
Another conventional method of 2D calibration improves the above issue by using the two control variables of SA compensation level and focus offset. The method selects a number of SA compensation levels and focus offsets, and performs focusing and tracking operations corresponding to each matching condition of the SA compensation level and focus offset. In this way, the optimal SA compensation level can be obtained by monitoring the performance of the HF jitter or push-pull amplitude. Therefore, the calculation result can approach the real optimal SA compensation level to simultaneously achieve better data quality and servo control.
However, the 2D calibration method, similarly, cannot read and calculate the HF jitters or push-pull amplitudes unless the tracking operation has been performed. As a result, not only much more time should be wasted in inefficient seeking and tracking operations, but also tracking failure may occur due to optical-disk shaking or improper settings of SA compensation level or focus offset in the calibration process. Besides, although the calculated SA compensation level can be very close to the real SA compensation level, the 2D calibration method requires too complicated procedures and thus largely increases the amount of data to be processed.